Bloodstained Monarchs can be found
on Islands 2-4 of the Dead Lands biome. So you'll first have to escape the first island to be able to find them. There is one monarch on each island.
The Bloodstained monarchs are the little guys trapped in a cage. You'll have to pay a hefty amount of gold to send an archer to the cage to shoot the rope and free them. Once freed, pay one (1) coin to switch your current monarch to the freed one. Free all four monarchs and the achievement should pop.
Pay attention: archers will only come if there is no Greed Portal between the trapped monarch and your base, so clear them out before you go for the monarchs.
Each monarch has a special ability. To activate it, press either
,
or
.
Monarch abilities:
*** Spoiler - click to reveal ***
Miriam (the character you start out with):
A whip lash that sends out a powerful wave which freezes the Greed and stops their movement for a short duration.
Zangetsu (red and pink guy): Raising his katana rallies the nearby Archers, Pikemen and Knights to increase their attack speed for a short time.
Alfred (green guy): Conjures a duplicate of himself and his steed that doubles his powers. The duplicate doesn’t take damage by the Greed or other hostile forest creatures.
Gebel (red guy with a cape): Gebel can take the form of a bat which allow him to fly fast and deceive the Greed. After a short period of time as a bat he reappears on his steed.